1. Field of the Invention
The present invention relates to burners for use in producing preforms that can be used to produce optical waveguide fibers either directly or through the intermediate production of a core cane.
2. Technical Background
In the production of optical waveguide fibers, conventional chemical vapor deposition methods such as vapor axial deposition (VAD) and outside vapor deposition (OVD) use source compounds with high vapor pressures, such as chlorides of silicon(SiCl4) and germanium (GeCl4). The source compounds are converted into vapor form using either bubblers or evaporators. The vapor is then transported into a flame and reacted with oxygen to form oxide soot particles. These particle are collected on a rotating starting rod or bait tube in the case of VAD or a rotating mandrel in the case of OVD. In some OVD systems, the cladding portion of the preform is deposited on a previously formed core preform or core cane, rather than on a mandrel.
In order for liquid or solution droplets to be converted into solid particles and then deposited on the target, the droplets must evaporate and combust with oxygen to form particles which are then captured on the target. The combustion, size and surface quality of the soot preform are dictated by the particle forming process and capture mechanisms.
Many of the high vapor pressure source compounds that contain elements which provide beneficial properties when incorporated into optical waveguide fibers are exceedingly difficult to make, are excessively expensive, and/or are difficult to handle. These drawbacks make it difficult if not impossible to effectively incorporate elements such as alkalis, alkaline-earths, and rare earths into the resultant optical waveguide fibers.
As an alternative to finding low vapor pressure compounds, and in order to generate sufficient vapor pressures from the compounds containing these beneficial elements, very high temperatures may be used within the burner. However, these elevated temperatures are not compatible with conventional vapor deposition equipment and the production of low loss fibers. An alternative way to deliver low vapor pressure compounds is to spray these compounds directly into the combustion zone in the form of liquid droplets. However, the spray often contains many large droplets that are not fully vaporized, resulting in large particle deposits on the target surface, due to their short resident time in the combustion zone.
Other problems are also associated with using these desired source compounds including the formation of large soot particles which create seed-warts, or imperfections, on the target surface. There are several mechanisms creating large soot particles (10 micrometers or larger) to form during droplet combustion. One mechanism is for non-vaporized large droplets to hit the target and continue to react on the target surface. Another source of large soot particles occurs when a droplet begins to evaporate, the vapor surrounding the droplet reacts with oxygen to form soot particles, and the remaining droplet serves as a nucleation center onto which the small particles aggregate, thereby forming a larger particle. Also, some solutions containing the desired compounds contain solid solutes involved in the reaction. When the solvent evaporates, solid solutes precipitate and decompose to form oxide particles. If the droplets containing the source solutes are large, the resultant oxide particles may also be large and serve as nucleation centers, thereby resulting in seed-warts being deposited on the target surface.
A solution is needed therefore which allows for depositing glass soot by forming liquid droplets of low vapor pressure compounds, increasing the resident time of the droplets in the combustion zone, and using the heat generated by combustion of fuel to vaporize these compounds.
One aspect of the present invention is to provide an apparatus for producing a glass soot that includes a first burner having a droplet-emitting first region, a gas-emitting second region surrounding the first region, and a gas-emitting third region surrounding the second region. The first region emits a glass forming mixture, the second region emits an inert gas, and the third region emits a combination of oxygen and a combustible gas. The apparatus further includes a conversion area having proximal and distal areas. The proximal area communicates with the first, second, and third regions. At least a portion of said glass-forming mixture is vaporized within the proximal area. The apparatus also includes at least two secondary burners each having gas-emitting fourth and fifth regions. The fourth regions of the secondary burners emit oxygen, and the fifth regions of the secondary burners emit a combustible gas. The distal area of the conversion area communicates with the fourth and fifth regions of the secondary burners. The glass-forming mixture is completely vaporized and converted to glass soot within the second section.
Another aspect of the invention is a method for increasing the resident time of low vapor pressure compounds in a combustion zone, including providing a first burner having a droplet-emitting first region, a coaxial gas-emitting second region surrounding the first region, and a coaxial gas-emitting third region surrounding the second region to define a conversion area having a proximate are and a distal area, wherein the proximate are is in communication with the first, second and third regions. The method also includes supplying a glass-forming mixture to the first region, an inert gas to the second region and a mixture of oxygen and a combustible gas to the third region, and vaporizing at least a portion of the glass-forming mixture by igniting the combustible gas within the proximate area of the conversion area. The method further includes positioning a second burner spaced from the first burner and having gas-emitting fourth and fifth regions, wherein the fourth and fifth regions are in communication with the distal area of the conversion area, supplying oxygen to the fourth region, and supplying a mixture of oxygen and a combustible gas to the fifth region. The method still further includes igniting the combustible gas within the conversion are for substantially completely vaporizing and converting the glass-forming mixture into glass soot, and forming a glass preform with the glass soot.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.
It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention.